The present invention relates to a phase locking apparatus and, more particularly, to a phase locking apparatus suitably applied to a synthesizer for synthesizing frequencies over the GHz band.
In general, a phase locking circuit is often used as a synthesizer for synthesizing an arbitrary frequency by using a voltage controlled oscillator, with the frequency of a reference oscillator serving as a reference, in various communication devices.
FIG. 1 shows a phase locking circuit formed as a synthesizer.
Referring to FIG. 1, a phase comparator 4 compares an output from a frequency divider 2 for frequency-dividing an output from a voltage controlled oscillator 1 into 1/n with an output from a high-precision reference oscillator 3, and outputs a signal corresponding to the phase difference between the output from the frequency divider 2 and the output from the reference oscillator 3.
The signal corresponding to the phase difference is supplied as a control voltage to the voltage controlled oscillator 1 through a low-pass filter 5.
In this case, a signal having a frequency n.multidot.f determined by an oscillation frequency f of the reference oscillator 3 and a frequency division ratio n is obtained from the output from the voltage controlled oscillator 1. Note that the output from the voltage controlled oscillator 1 may be extracted through a buffer amplifier 6, as indicated by the dashed line.
When such a phase locking circuit is to be formed into an MIC (Microwave Integrated Circuit), the voltage controlled oscillator 1 and the frequency divider 2 are formed by different integrated circuit chips to be optimized to prevent the characteristics of these components from affecting each other. The output power of the voltage controlled oscillator 1 is divided into two components. One component is guided to the output side, and the other component is input to the frequency divider 2.
With this arrangement, in a phase locking circuit which operates at a high frequency in the GHz band or higher, in particular, an output from an voltage controlled oscillator 1 is distributed to the output side and a frequency divider 2 through a hybrid circuit 7 or power divider, and power matching is performed on the input side of the frequency divider 2, as shown in FIG. 2.
In this arrangement, however, the output power of the voltage controlled oscillator 1 must be set to be larger than that actually required as an output by the power component input to the input terminal of the frequency divider 2, resulting in a decrease in efficiency.
In addition, if the hybrid circuit 7 is used as an element for distributing an output from the voltage controlled oscillator 1 to the output side and the frequency divider 2, the element size becomes large. If a power divider is used, the power loss increases. As a result, the output power of the voltage controlled oscillator 1 must be further increased.
That is, in the conventional phase locking apparatus, since the output power of the voltage controlled oscillator must be distributed to both the output side and the frequency divider, the output power must be set to be large. If a hybrid circuit is used for power distribution, the element size increases. If a power divider is used, the power loss increases.